This application claims the benefit under 35 U.S.C. §119(e) of prior U.S. Provisional Patent Application No. 61/315,607, filed Mar. 19, 2010; prior U.S. Provisional Patent Application No. 61/352,521, filed Jun. 8, 2010; and prior U.S. Provisional Patent Application No. 61/365,658, filed Jul. 19, 2010; which are incorporated in their entirety by reference herein.
The present invention relates to processes to control the growth of bacteria in fermentation processes with antibiotic alternatives. More particularly, the present invention relates to processes for fermentation to produce ethanol with bacterial control using antibiotic alternatives which include at least one nonoxidizing biocide and/or at least one stabilized oxidizer.
Worldwide demand for industrial ethanol is increasing in view of its utility as a fuel or fuel supplement, e.g. in admixture with gasoline, and because of its availability from numerous renewable sources and waste materials.
Ethanol can be produced by fermentation using a wide variety of starch containing raw materials. Starch-based ethanol production generally includes preparing a mass of starchy feedstock that contains or can be degraded into fermentable sugars, adding water to make a mash, saccharification of cellulose or other complex carbohydrates into fermentable sugars, and adding yeast which ferments the sugar into ethanol and carbon dioxide. Ethanol is recovered by subjecting the fermented mash to distillation. A co-product of distillation in ethanol production is non-starchy solids containing proteins, fibers, and oils, which may be processed to produce “distillers dried grains with solubles” or “DDGS”. DDGS are nutrient-rich and are commercially sold as an animal feed, feed supplement, or plant fertilizer.
A problem in the ethanol production industry is that ethanol fermentation process equipment and/or the mash can become contaminated with bacteria that reduce production yields. “Lactic acid bacteria” is one class of bacteria that poses a problem in this respect. Lactic acid bacteria include, for example, Lactobacillus, Pediococcus, Leuconostoc and Weissella species. Acetic acid bacteria, e.g., Acetobacter sp., can also cause problems by producing acetic acid or other organic acids which foul the process and reduce the yields of ethanol. Yeast converts sugars to ethanol, but bacteria also convert those same sugars to make lactic or acetic acid instead of ethanol, leading to reductions in ethanol production yield. To control the outbreak of such bacteria, antibiotics have been used in bioethanol fermentation processes. The antibiotics used for these treatments may include, for example, virginiamycin, penicillin, erythromycin, and tylosin. These antibiotics also are used in veterinary and human medicine. The risk of the bacteria developing drug-resistance to antibiotics from their use or overuse is a known and growing concern. Switching antibiotics or increasing antibiotic dosages may not provide a long-term solution and may compound the antibiotic resistance problem. Further, questions have been raised about non-specificity of the antibiotic to the target bacteria and fermentation products. Concerns also have been raised about the presence of antibiotic residues in the DDGS destined for animal feeds. Stricter legislative and regulatory controls on the use of antibiotics in ethanol fermentation applications may be enacted. Alternatives to antibiotics are needed for ethanol fermentation processes.
Chlorine dioxide (i.e., ClO2) has been proposed as an oxidizing biocide. However, chlorine dioxide is a strong oxidizing agent which has nonselective antimicrobial action. Chlorine dioxide attacks both unwanted bacteria and yeast crucial to the fermentation process. Loss of yeast translates into loss of ethanol yield and/or a “sluggish” fermentation and/or a “stuck” fermentation. Chloride dioxide also generates chloride ions, which can corrode equipment and lead to iron deposits or pitting in the process equipment, as well as release iron and chromium into the process system, which can require costly repairs.
The present investigators have recognized a need for ethanol fermentation strategies that can displace antibiotics for bacterial control with minimal adverse environmental impact of their own.